1. Technical Field
The present invention relates to a screen on which an expanded display image produced by a projector is projected, and more particularly to a screen unit which is easy to assemble and has high flatness sufficient for displaying an image projected by a projector without distortion.
2. Related Art
Recently, a close projection type (short focus type) projector which projects an image from a position close to a screen has been put on the market, and the chance of projecting an expanded image on a screen by using this type of projector has been increasing. This close projection type projector which can be placed at a position close to a screen unlike a related-art projector which projects an image from a position away from the screen and thus occupies a wide area on the desk, for example. Moreover, a person who goes across the front of the projector and thus becomes an obstacle for projection of an image in case of the related-art projector does not pass through the front of the close projection type projector. Therefore, the close projection type projector has high usability.
According to the close projection type projector, however, the projected image is largely distorted when the surface of the screen has low flatness and has a wavy shape or other non-flat shapes. Incase of the related-art projector placed at a distant position from the screen and projecting an image approximately from the front, the incident angle of the projection image with respect to the normal line of the screen surface is small. Thus, the quality of the projection image scarcely deteriorates even when the screen has low flatness. According to the close projection type projector, however, the large incident angle of the projection image produces distortion on the projection image when the screen has low flatness, which considerably deteriorates the image quality.
JP-A-2006-154854 and JP-A-2000-131766 disclose examples of the screen having low flatness. According to screen units shown in JP-A-2006-154854 and JP-A-2000-131766, two sides of a rectangular screen different from two sides fixed by supporters are unfixed and free. The two unfixed free sides decrease the flatness of the screen when they are curled. Moreover, according to the technologies disclosed in JP-A-2006-154854 and JP-A-2000-131766, the supporters apply tension only in the direction of separating the fixed and opposed two sides away from each other. In this case, the internal stress within the plane of the screen does not become uniform, and the flatness decreases in a region where the internal stress is small and a region where the internal stress sharply changes. When the close projection type projector projects an image on a screen having low flatness as the screens disclosed in these references, the image quality considerably deteriorates.
JP-A-3-282442 proposes a technology which increases the flatness of a screen by tensioning all the four sides of the screen. However, this screen unit has a complicated structure and is difficult to assemble by a manufacturer or a user. Furthermore, according to the screen unit disclosed in this reference, expanding and contracting mechanisms using springs are included in supporters, which lowers the rigidity of the supporters. In this case, stresses are difficult to be uniformly given to the entire surface of the screen base material, and thus the flatness of the screen lowers.